In the process described in application Ser. No. 188,254, the contents of said application incorporated by reference herein, an aqueous solution of magnesium hydroxide is used to remove sulfur dioxide from a flue gas stream, in a wet scrubber. Spent solution or effluent from the scrubber is subjected to an oxidation step. In an oxidizer, air is sparged through the solution to convert sulfites to sulfates. The oxidized product, containing magnesium sulfate, is treated or regenerated by addition of a magnesium-containing lime slurry to precipitate calcium sulfate (CaSO.sub.4.2H.sub.2 O) from the aqueous media and form a magnesium hydroxide (Mg(OH).sub.2) suspension, while magnesium hydroxide is returned to the scrubber and separated calcium sulfate precipitate, or gypsum, is removed from the system for use or disposal. While this process is acceptable, certain problems exist. For example, in such a process, it is somewhat difficult to separate or remove calcium sulfate from the system because of the dewatering properties of the calcium sulfate-containing aqueous media. Also, in such a process, the gypsum product may contain magnesium hydroxide, which is an objectionable component in the primary uses to which gypsum is put. In addition, it is difficult in such a process to control the pH value in the oxidation step, with sometimes, release of sulfur dioxide occurring from the oxidizer unless care is taken.
It is an object of the present invention to provide a scrubbing method for flue gases, using a magnesium hydroxide scrubbing system, that will provide a sludge that is more readily dewatered.
It is another object of the present invention to provide a scrubbing method for flue gases, using a magnesium hydroxide scrubbing system, where a higher percentage of the total amount of gypsum and magnesium hydroxide are removed from the sludge discharged from the system than with previous such processes.
It is a further object of the present invention to provide a scrubbing method for flue gases, using a magnesium hydroxide scrubbing system, which provides a more saleable gypsum from a magnesium hydroxide scrubbing process, while also recovering a higher amount of magnesium hydroxide from the process.